1. Field of the Invention
The present invention relates to improved copoly(carbonate/phosphonate) compositions which are useful thermoplastic materials for forming molded and shaped articles.
2. Description of the Prior Art
Polycarbonate resins are a well-known type of engineering thermoplastic formed by condensation polymerization of a dihydric phenol, such as, bisphenol A, and a carbonate precursor, such as, phosgene. They are useful in the formation of molded and shaped articles having good impact resistance and strength. It is also known to include an organophosphorus dihalide as a third major monomeric reactant in the reaction medium to form copoly(carbonate/phosphonate) compositions. In such compositions, the basic polycarbonate structure is modified by the presence of phosphonate structures derived from the organophosphorus dihalide reactant, and this phosphonate structure contributes to the flame retardancy of the resulting polymer. Representative copoly(carbonate/phosphonate) compositions that are known to the prior art are described in the following patents and publications: Belgian Pat. No. 610,954; British Pat. No. 1,031,481; U.S. Pat. No. 3,378,523; U.S.S.R. Pat. No. 180,336; Soobshch. Akad. Nauk Gruz. SSR, 1971, 61(2), 317-319; and Vysokomol. Soedin., Ser. B 1970, 12(5), 384-387.
Although the additional presence of the organophosphorus dihalide contributes to giving greater flame retardancy to the resulting polymer, as compared to conventional two-component polycarbonates, such compounds, when used as a third monomeric reactant, tend to result in a decrease in the melting point and inherent viscosity of the resulting copoly(carbonate/phosphonate) composition as compared to a two-component polycarbonate not containing the organophosphorus dihalide. Reduction of the melting point and inherent viscosity is undesired since it detracts from the desired toughness of the endproduct.